This invention relates to low noise faulted current indicators, and particularly to methods and means for indicating the existence and locations of faults in such systems.
Faulted current indicators (or FCI's or fault indicators) are placed at intervals along power lines or cables to decrease the time of locating faults, either temporary or permanent, on utility distribution systems. In general, each FCI is a high current trip and low current reset indicating device that displays whether a fault has occurred between the indicator and a load. Such devices often respond unintentionally to noise in neighboring power lines or cables and may therefore provide inaccurate information.
As long as a fault indicator detects only the currents in the monitored line, it displays a fault predictably and resets reliably. However, extraneous magnetic fields affect the fault indicator, especially where the line it monitors is near other electrical lines; as for example when the line that the fault indicator monitors is one phase conductor of a three phase line in the close vicinity of another phase conductor. The currents through the other phase conductors generate their own magnetic fields which induce noise signals in the fault indicators. Such noise is usually not sufficient to trip the fault indicator while it is sensing current in its phase conductor. However, the noise may reach values of sufficient amplitude to reset a tripped fault indicator when no current flows in the line. The would then fail to announce a fault, and line crews would find it more difficult to locate the fault and repair it. Since most indicators exhibit this susceptibility, a substantial portion of the indicators in a faulty line may become reset and ineffective.